In optical design, one optical system may be followed by another optical system. In this situation it can be important to efficiently couple light from one to the other. This can be accomplished by, for example, using three considerations.
First, the working f-number for light exiting the first system can be matched to the working f-number for light entering the second system, which can help allow an axial bundle of light to be coupled efficiently. Efficient coupling may relate to cost and light throughput. If a first system has a small f-number and a second system has a larger f-number, then some light may be lost by joining these together. If a first system has a large f-number and the second system has a lower f-number, then no, or substantially no, light is lost but the second system is “too expensive” because it is overdesigned to work at a larger aperture size. Thus, one way is to match the two systems. A second condition for optimizing efficient coupling of light between optical sub-systems is to ensure that the location of the exit pupil of the first system can be the same as the location of the entrance pupil for the second system, which can help off-axis bundles of light to be coupled efficiently. Lastly, any vignetting that is done in the first system can be matched by similar vignetting in the second system. Vignetting can be used to change the effective f-number as a function of field position. Assuming that pupil locations and vignetting are details that are taken care of in the optical design and assuming the system has circular symmetry then any one of, working f-number, NA, cone angle or f-number (at infinite conjugate) can be used as the matching criteria.
When the aperture stop is no longer circular (e.g., when pupil masking is used), the previous terms are no longer constants but vary as a function of an angle. However there still exists a maximum radius at some angle that corresponds to a maximum cone angle and NA and a minimum working f-number or f-number. The optics can be designed to work under this condition and so the same matching criteria can be used as before. However, one additional condition may be imposed that is the shape and orientation of the exit pupil of the first system matches the shape and orientation of the entrance pupil of the second system.
Often the principle of matching working F-number is used in the design of the projection and illumination optics in projectors. In particular, this is used in DLP cinema projectors, which use the digital micromirror device (DMD) provided by Texas Instruments Inc. of Dallas, Tex. DMD spatial light modulators have been successfully employed in digital projection systems, including digital cinema devices that meet the DC2K digital cinema resolution standard. Efficiency measurements have been performed on such systems that use the DC2K chip and have found the efficiency optimizing principles to be valid; however, for systems utilizing the new DC4K chip the principle was not found to work as well.
Separate from the development of digital projection technology, including DMD and LCOS based projectors, laser projection technology has been evolving on a largely independent path, paced by the development of the lasers. One example is the system described in the paper “Laser Digital Cinema Projector,” by G. Zheng et al., Journal of Display Technology, Vol. 4, pp. 314-318 (2008), which retrofits lasers into a conventional 2K DMD based digital cinema projector. A second exemplary system, described in “A Laser-Based Digital Cinema Projector”, by B. Silverstein et al. (SID Symposium Digest, Vol. 42, pp. 326-329, 2011), describes a laser projector using 2K DMD spatial light modulators and custom optics.
In support of the development of laser projectors, transmission experiments were performed using red, green, and blue lasers. Unacceptable efficiency losses were observed with various combinations of laser wavelengths and 4K DMD devices. These losses may be due to increased diffraction from the finer pitch of the DC4K chip. Diffraction occurs when propagating waves (e.g. light waves) encounter an obstacle and its behavior is modified. This can happen, for example, when the size of the obstacle is similar to the wavelength of the wave and when the obstacle includes multiple, closely-spaced openings. This can also result in a complex spreading of the distribution of light not predicted from geometrical optics.
To improve the transmission of the red light, the size of the projection aperture stop may be increased. However, opening the projection stop can have the undesirable effect of lowering sequential contrast ratio. Sequential contrast ratio is the value obtained by measuring the brightness at a spot of a full white image divided by the brightness at the same spot of a full back image. The significant gain in red transmission may be more desirable than the modest decline in red contrast ratio. Since the aperture sizes are already correct for blue and green light, opening the projection aperture may result in a decrease in blue and green contrast. This can be undesirable especially since the majority of the luminous flux is in the green channel. Thus, there is a need for a color dependent aperture to maximize red transmission but at the same time not decrease contrast for green and blue light.
U.S. Pat. No. 7,400,458 to M. Farr provides a projection system having “wavelength dependent aperture stops” in the illumination sub-system, in which patterned thin film coatings are provided on a substrate. The resulting concentric ring spectrally dependent aperture stops modify color channel light levels and improve image quality. U.S. Pat. No. 7,321,473 to C. Liu provides a projection lens having a lens aperture where spectral filters provide concentric ring spectrally dependent apertures for the purpose of improving image resolution on a color dependent basis. Similarly, U.S. Pat. No. 7,008,065 to R. English et al. provides color balancing aperture stops or apodizing aperture stops in either the illumination system or projection optics. In this case, both concentric ring spectrally dependent apertures and shaped aperture stops designs are used, with the goal of improving illumination light levels for color balance tuning and setting white point, as well as improving image contrast from an LCOS light valve.
Although projectors, including digital cinema projectors, using the digital micro-mirror devices from Texas Instruments are in commercial use, it has not been widely recognized that the diffraction behavior of the DMD devices has changed as the features have become smaller. As such devices are used with narrow band light sources, including lasers, diffraction effects will become increasingly important and it is no longer sufficient to treat these micro-mirrors as simple reflective devices. Therefore, an opportunity remains to further improve the design of projectors using micro-mirror array modulators such as the DMD devices, including through the use of optimized color dependent apertures.